The invention relates to a process for the recovery as a feedstock of an alpha-olefin from a mixture containing mainly hydrocarbon compounds.
In a published concise rendition of a lecture: C L Render, Z Denga xe2x80x9cSasol Alpha Olefinsxe2x80x9d from the lecture series xe2x80x9cNew processes in chemical technologyxe2x80x9d at the ACHEMA Congress 1994, a process is described which permits the recovery of alpha-olefins and more particularly 1-pentene and 1-hexene in relatively pure form from the liquid fraction produced in the Fischer-Tropsch-Synthesis. Since a large number of other compounds of various types boil in the immediate vicinity of the boiling temperature of the desired alpha-olefins, of which some even form azeotropic mixtures with the desired alpha-olefins, and although the distillative separation of interfering contaminants represents an important process step, it is nevertheless not possible with acceptable effort to attain the desired purity of the alpha-olefin product solely by distillation.
Accordingly, tertiary olefins are first converted with methanol into ethers in a pentene or hexene fraction pre-concentrated by distillation. This etherification step is followed in a so-called superfractionation by a sharp distillative separation of all the components which are either lower or higher boiling than the desired alpha-olefin. Jointly with the lower boiling components excess methanol derived from the etherification step is also separated. From this low boiling fraction the methanol is extracted with water and is recovered from the aqueous extract by a distillative methanol/water separation.
Jointly with the higher boiling components the ethers formed in the etherification step are likewise separated.
The alpha-olefin crude product of superfractionation has a typical purity of about 90%. It now only contains components which boil very close to the desired alpha-olefin and which, therefore, can be separated by normal distillation only with extremely great efforts. However, all these interfering components differ from desired alpha-olefins by their polarity and may, therefore, be separated relatively easily in the polar medium of the extractive distillation succeeding the superfractionation. The extractive distillation is performed with aqueous NMP (N-methyl-2-pyrrolidon) as a solvent. Accordingly, the alpha-olefin final product must still be dried. This is done by a distillative separation of the water.
An improvement of this known process for the recovery of alpha-olefins was filed as a patent application at the German Patent Office under file number 197 23 049. An important integer of this improved process resides in the provision of the etherification reactor downstream of the distillative fine separation of all lower boiling components (lower boiling than the desired alpha-olefin).
Due to a reaction equilibrium arising in the etherification reactor, it is not possible to attain a complete conversion of the tertiary olefins into the corresponding ethers. Depending on the original content of tertiary olefins there accordingly arises a maximum attainable purity of the alpha-olefin product, hardly exceeding 99%xe2x80x94even after practically complete removal of all other impurities. (Here and in what follows the purity is stated in percentages by mass). This applies to the process according to C L Render and Z Denga as well as to the improved process (197 23 049.0).
The process described by C L Render and Z Denga is well suited to recover from the fractions obtained in the coal liquefaction according to Fischer-Tropsch, e.g. 1-hexene with a purity of 98.5%. Higher purities can only be attained with progressively increasing yield losses. The reason therefor resides in the tertiary olefins accompanying the feedstock and the boiling points of which are close to 1-hexene and which, therefore, can virtually not be separated off by conventional separating methods. These tertiary olefins must be converted in the reactor to separable compounds. In a singe-stage reactor the degree of conversion is limited, however, by the chemical equilibrium so that the maximum attainable purity of the 1-hexene product is ultimately limited by the reaction equilibrium.
In order to overcome this limitation regarding attainable purity to about 99% a process is proposed in DE 198 25 295 A1.
From a mixture containing predominantly hydrocarbon compounds, as obtained in Fischer-Tropsch-Synthesis, alpha-olefins are recovered after an at least crude separation of components boiling higher and/or lower than the alpha-olefin. For that purpose tertiary olefins of the mixture boiling close to the alpha-olefin to be recovered are subjected to a catalytic etherification after a superstoichiometric addition of a low alcohol. A stream of ethers and other high boiling reaction products recovered from the etherification together with the alpha-olefin is forwarded to a distillative separation of components boiling higher than the olefin.
The etherification is performed in multiple stages, at least in two stages. After each etherification step etherification products are separated as residue streams, where applicable jointly with other higher boiling components.
In addition, it is proposed to separate the low boiling alcohol as an azeotropic mixture after the etherification and separation of the etherification products in a further distillation step and to subject the olefin fraction so obtained when desired or required to further separation steps, e.g. an extractive distillation and/or an adsorption.
The process disclosed in DE 198 25 295 A1 is directed at recovering by the same method a variety of alpha-olefins, in particular, however, 1-hexene or alternatively 1-pentene having a purity in excess of 99%.
Although the problems of purifying e.g. 1-hexene or 1-pentene are similar, they are not identical. Accordingly, the process according to the state of the art is not equally suitable for the recover of whatever alpha-olefin.
Accordingly, a need exists for a more simple process than that in the prior art for the recovery of 1-pentene and alpha-olefins boiling lower than 1-pentene.
The invention provides a process for the recovery as a feedstock of an alpha-olefin from a mixture containing mainly hydrocarbon compounds, such as is obtained by Fischer-Tropsch-synthesis after an at least crude separation of components boiling higher and/or lower than the alpha-olefin, wherein tertiary olefins of the mixture, after super-stoichiometrical addition of a low alcohol, are subjected to catalytic etherification and a stream derived by etherification is fed jointly with the alpha-olefin and the ethers produced and other high boiling reaction products to a distillative separation of components boiling higher than the olefin.
Thus, according to a first aspect of the invention, there is provided a process for the recovery as a feedstock of an alpha-olefin from a mixture containing mainly hydrocarbon compounds, such as is obtained by Fischer-Tropsch-synthesis after an at least crude separation of components boiling higher and/or lower than the alpha-olefin, wherein tertiary olefins of the mixture, after super-stoichiometrical addition of a low alcohol, are subjected to catalytic etherification and a stream derived by etherification is fed jointly with the alpha-olefin and the ethers produced and other high boiling reaction products to a distillative separation of components boiling higher than the olefin, in which from the mixture, 1-pentene or an alpha-olefin lower boiling than 1-pentene is recovered, wherein
after a distillative fine separation of substances lower boiling than the alpha-olefin to be recovered from the mixture the etherification is performed with excess methanol in a single or a plurality of stages;
during the distillative separation of the components higher boiling than the alpha-olefin from the stream derived by the etherification a by-product is separated, comprising ethers and other components higher boiling than the alpha-olefin and a stream comprising the alpha-olefin and the excess methanol is recovered and subjected to an extractive distillation with methanol as solvent; and
the excess methanol is recovered approximately completely during the extractive distillation and is returned to the etherification stage, supplemented by a methanol feed stream.
By the limitation to the recovery of 1-pentene or an alpha-olefin boiling lower than 1-pentene and to mixtures, in which prior to the etherification the components boiling lower than the alpha-olefin are separated by fine separation, a low throughput in the process steps downstream of the fine separation is attained and a simplified overall process including less separation effort.
As a result of the excess methanol from the etherification being reused in the extractive distillation performed downstream, it is possible to perform the etherification with a virtually unlimited methanol excess. This results in a more complete etherification than in the process according to the state of the art. As a result it is in many cases possible to dispense with a second etherification step. The reactor required for the etherification may, due to the fine separation performed upstream and the more effective etherification, be restricted to small dimensions.
Due to the fact that in the etherification methanol is employed as a reactant and downstream methanol is also used as a solvent in the extractive distillation, a separate infrastructure for the provision of a solvent for the extraction, as is required in the process according to DE 198 25 295 A1, can be dispensed with.
The recovery of the excess methanol in the course of the processing of the solvent for the extractive distillation which is necessary in any case and the reuse of the methanol during the etherification jointly with an added methanol feed stream, reduces the amount of methanol to be employed to just little more than the amount which is used during the etherification.
In an embodiment of the process according to the invention the stream including the olefin and the excess methanol are fed into an absorber of the extractive distillation in order to separate oxygenates and dienes, wherein methanol is used as a solvent and the remaining gaseous olefin stream is subjected to scrubbing with water in order to remove methanol.
The scrubbed olefin stream may be subjected to a distillative drying step.
In order to be dried, the scrubbed olefin stream may be cooled, olefins and water be condensed, be separated by decanting, the water fraction be fed into the water scrubbing stage for methanol removal and dissolved water may be extracted from the liquid olefin fraction by means of a heated drying column and a dry olefin production stream may be recovered.
Firstly, a liquid methanol stream including the oxygenates, dienes and a little olefin derived from the heated sump of the absorber and secondly a methanol/water mixture virtually free of oxygenate and diene derived from the lower part of the water scrubbing stage of the olefin may be passed to a methanol stripper for purposes of regeneration.
By means of the methanol stripper it is possible to produce a sump product, comprising virtually methanol-free water, a virtually oxygenate-free and diene-free methanol stream including a little water may be withdrawn in a first side outlet and be used in the absorber as a solvent, and a virtually water-free methanol stream containing little oxygenate and diene may be withdrawn in a second side outlet and returned to the etherification.
In a section of the methanol stripper above the second side outlet oxygenates and dienes may be enriched in the rising gas and be passed by way of a chimney tray into a head portion of the methanol stripper, further methanol being scrubbed out in the head portion by means of water scrubbing, and a virtually methanol-free overhead gas be recovered jointly with the oxygenates and dienes and the scrubbing water with the methanol dissolved therein be forwarded to a lower section of the methanol stripper.
The virtually methanol-free overhead gas may be condensed, a water fraction and a fraction including the oxygenates and dienes being formed by decanting, the water fraction being used for water scrubbing in the head portion of the methanol stripper and the fraction including the oxygenates and dienes being fed partly below the chimney tray as a reflux liquid and partly being fed as a by-product stream to some further use.
The methanol-free water from the sump of the methanol stripper can be used in one portion in the water scrubbing in the head portion of the methanol stripper and in another portion in the water scrubbing of the olefin stream.
In an alternative embodiment of the process according to the invention, there is fed firstly from the heated sump of the absorber a liquid methanol stream including the oxygenates, dienes and a little alpha-olefin to a methanol regenerating column and secondly an almost oxygenate and diene-free methanol/water mixture from the lower part of the water scrubbing stage of the olefin to a methanol/water separating column.
By means of the methanol/water separating column it is possible to produce almost methanol-, oxygenate- and diene-free water as a sump product and, in addition, to withdraw an oxygenate- and diene-containing methanol overhead product, condensed and used in one portion as a solvent in the methanol/water separating column and in another portion be fed into the methanol regenerating column.
From the sump of the methanol regenerating column and almost oxygenate- and diene-free methanol stream may be withdrawn, one portion thereof being used in the absorber as a solvent and another portion being returned to the etherification step.
In the methanol regeneration column oxygenates and dienes may be enriched in the rising gases and be passed by way of a chimney tray into a head portion of the methanol regenerating column, further methanol being scrubbed out by water scrubbing in the head portion and a virtually methanol-free overhead gas being recovered with the oxygenates and dienes and the scrubbing water with the dissolved methanol being passed into the methanol/water separating column.
The virtually methanol-free overhead gas is advantageously condensed, by decantation a water fraction and a fraction containing the oxygenates and dienes are formed, the water fraction is used for water scrubbing in the overhead portion of the methanol regenerating column and the fraction comprising the oxygenates and dienes is in part fed below the chimney tray as reflux liquor and for another part is passed as a by-product stream to further use.
The methanol-free water from the sump of the methanol/water separating column may in part be used in the water scrubbing in the head portion of the methanol regenerating column and to another part in the water scrubbing of the olefin stream.